Automatic Exposure Control (AEC) apparatus are widely used in conventional diagnostic X-ray equipment to control X-ray exposure levels received by a patient. Using an AEC device can help to limit the amount of radiation that is received by sensing the radiation level at a suitable location in the exposure path and providing an output signal that indicates when sufficient radiation has been received. This output signal is then used to disable power to the X-ray emission components, thereby stopping the generation of ionizing radiation.
The schematic block diagram of FIG. 1A shows an X-ray imaging system 10 that is used to provide a radiographic image of a patient or other subject 14. When the technician operates a control 24, an X-ray source 16 generates the ionizing radiation that is used for exposure and for forming an image onto a detector 12. An Automatic Exposure Control (AEC) apparatus 20 has one or more sensor elements 22 that respond to incident radiation by generating a signal that indicates the amount of radiation received. A generator control 18 interprets this signal and responds to terminate x-ray emission at an appropriate point.
AEC sensor elements 22 are typically located at suitable locations just behind the patient or other subject 14 in order to sense the amount of radiation received over particular areas of subject 14. Sensor elements 22 may be individual sensor elements, or may be integrated into a panel that is positioned behind the patient, as suggested in FIG. 1A. In other embodiments, sensor elements 22 of the AEC apparatus 20 are provided on the surface of detector 12 or in the bucky or other holder that is used for retaining detector 12.
The basic schematic diagram of FIG. 1A can be used with any type of X-ray detector technology, that is, with film, with computed radiography (CR) plates, or with a digital radiography (DR) flat panel detector.
The plan view of FIG. 1B shows a conventional arrangement of AEC apparatus 20 having three sensor elements 22. In conventional use, AEC sensor elements 22 are in fixed positions in front of the X-ray detector 12; in some systems, detector 12 or a plate holding AEC sensor elements 22 can be rotated within the plane in order to position the sensor element 22 devices appropriately with respect to the patient. Signals from individual sensor elements 22 are collected and combined for transmission to generator control circuitry.
The use of a standard pattern of AEC sensor elements 22 in fixed positions, as shown in FIG. 1B can present some problems. AEC sensor elements 22 work best when placed behind the area of bone or tissue that is of most diagnostic interest. This area can differ from one patient to the next. In addition, patient body size and proportions vary over a range, so that no one fixed pattern of AEC sensor elements 22 works optimally for all patient sizes and for all imaging situations. Some compromise is made for imaging under particular conditions when using the conventional AEC arrangement.
In some conventional x-ray systems, one or more AEC sensor elements 22 can be disabled for a particular image, allowing the operator to compensate somewhat for differences in the anatomy being imaged or for patient positioning. However, this solution can mean less accurate detection of the exposure level and risks over- or under-exposure for obtaining the image of the patient or other subject.
There are a number of inherent problems with using conventional AEC apparatus. One problem relates to obstruction of a portion of the image signal. As shown in the example of FIG. 1A, the AEC sensor elements 22 are disposed in the imaging path, prior to detector 12. Even though AEC sensor elements 22 are fabricated of low-density materials that reduce their interference with signal content, some amount of signal degradation occurs due to the AEC device. Another problem relates to fixed positioning; AEC sensor elements 22 are not always in the best position for obtaining an image due to differences in types of anatomy imaged, patient size, and, particularly with portable radiography apparatus, variable detector 12 positioning relative to subject 14 and to the AEC sensor elements 22.
Thus, it can be seen that a more flexible arrangement for AEC sensing would have advantages for adapting to different patients and to different imaging applications.